As known in the art, a coaxial cable is formed of two concentric conductors separated by a dielectric. This unique construction results in the restriction of the electromagnetic field to the region between the inner and outer conductors, which results in near perfect shielding between fields inside and outside the cable.
Coaxial cables are generally used to propagate high-frequency signals from one electrical device to another. Generally, both electrical devices can be at the same ground potential. However, some applications, for example large systems that utilize both high and low frequency signals, may be susceptible to low frequency noise (e.g., approximately 1 kHz and below) caused by ground loops. In this case, it is desirable to break up potential ground loops. One way to do this is to break the ground connection in the coax line. For example, in industrial RF semiconductor testers, which require testing in both high and low frequency ranges (e.g., digital, low frequency analog, RF, etc.), the RF signals are generated in a separate rack and connected to the semiconductor test interface by way of one or more coaxial cables. The RF rack is tied to protective earth through the AC power connection or communications link. The semiconductor test interface may also be tied to protective earth through the handler (a device which automatically places the semiconductor onto the tester), AC power connection or communications link. Thus, the coax connection between the RF rack and semiconductor test interface may complete a ground loop between the RF rack and digital tester which can introduce low frequency noise. In this case, it is desirable to break the ground loop by breaking the coax connection at low frequencies where ground loops are an issue.
However, such a configuration is problematic. Even when two devices are both grounded though a common power connection or other means, the ground potential of each is slightly different depending on the electrical length and impedance of the connections. When one electrical device (or portion thereof) is grounded at one potential and the other electrical device is grounded at a different potential, the noise potential of the devices is different in magnitude and phase. Thus, when connected by way of a coaxial cable with a DC block, at low frequency a discontinuity exists in the ground on either side of the DC block. Due to this discontinuity, the ground noise potential on either side of the DC block is different. This results in noise being introduced into the system.
Accordingly, system designers have attempted to build a DC block which prevents DC current flow along the coaxial cable while permitting RF power to flow through the DC block. The general scheme in achieving this goal is to cut the coaxial cable, and then capacitively couple the two lengths of coaxial cable together with a capacitance that has a high impedance at DC and thus breaks up ground loops, yet effectively couples signals at higher frequencies. This solution is problematic due to the coaxial configuration of the coaxial cable transmission line. Although the insertion of a capacitor between the two inner conductors of the two lengths of coaxial cables is straightforward, the insertion of a capacitance between the two outer conductors of the two lengths of coaxial cables is problematic. The insertion of a capacitor between the two outer conductors of the two lengths of coaxial cables generally degrades the shielding characteristics of the coaxial cable and adversely affects the integrity of signals propagated through the coaxial cable.
Ideally, a DC block should have very low impedance on the outer conductor in the desired frequency range of signal propagation, and high impedance in the very low frequency range in order to break up ground loops. Of course, the actual values of these frequencies will depend on the application.
Although some DC blocks have been developed which capacitively break the outer coax connection, to date, these DC blocks do not have low enough impedance when the desired signal propagation frequency range includes lower frequencies (but greater than the very low frequencies seen on ground loops). Greater impedance at low frequencies can introduce low frequency noise on the propagated signals. In order to decrease the frequency at which the impedance of the outer connection begins to increase, the coupling capacitance needs to be dramatically increased in a way such that the impedance is very low across the continuous frequency band (no resonance points). In addition, the microwave structure needs to be maintained and the structure cannot be exposed to outside interference. In the prior art DC blocks, the outer connection is limited in capacitance due to its construction.
Accordingly, a need exists for a DC block that blocks very low frequency signals with high impedance, yet, at higher frequencies, maintains the electric field cancellation effect of standard coaxial transmission lines through the DC block.